1. Field of the Invention
This invention concerns a method of regenerating hydroprocessing catalyst. More particularly, this invention relates to a means of regenerating hydroprocessing catalyst in a reduced time period as compared with prior regeneration procedures.
2. Description of the Prior Art
Hydrocracking may be regarded as a combination of cracking, hydrogenation and isomerization. It may also be described as a treating operation, as hydrogen combines with, and practically eliminates, contaminants in the feed such as sulfur and nitrogen. Typical hydrocracking catalysts have both a cracking component, such as silica alumina, and a reforming agent such as platinum, nickel and/or tungsten oxide.
Commercial hydrocracking processes are operated at temperatures between about 400.degree. and 800.degree. F and at pressures from about 100 to 3,000 psig. Operating severity and hydrogen consumption are dependent upon feedstock and the particular product distribution required, as well as the process itself and the catalyst used. A typical process flow consists of mixing hydrogen with a refractory gas oil derived from cracking and/or coking operations, or a virgin or heavy gas oil. This mixture of hydrogen and gas oil is then heated and contacted with catalyst in a fixed-bed reactor at a specified hydrogen pressure. Most feedstocks require pretreatment; in some processes this is the first step in the two-stage system.
As a result of complex hydrocarbon processing reactions in a hydrocracker reactor, a carbonaceous deposit is laid down on the catalyst which is referred to by petroleum engineers as "coke". This deposit of coke on the catalyst tends to seriously impair the catalyst efficiency for the principal reaction desired and to substantially decrease the rate of conversion and/or the selectivity of the process. The purpose of regenerating catalyst (whether a one or two stage cracking operation) is to restore catalyst activity by removing coke and other impurities laid down during normal operation. This is accomplished by contacting the catalyst with a dilute oxygen-inert gas mixture at controlled temperature in order to burn off impurities. Metal sulfides on the catalyst are converted to oxides with the liberation of sulfur oxides (SO.sub.2 and SO.sub.3). The use of dilute oxygen limits the temperature rise from the heat of combustion.
The time between replacements of hydrocracker catalyst can be significantly increased, with resultant savings in catalyst cost and plant downtime through the use of in-place regeneration.
A conventional hydrocracker catalyst regeneration employs several "burns" (generally at least 3) wherein dilute oxygen is contacted with the catalyst. The oxygen supports combustion of the coke and forms CO and CO.sub.2 as oxidation products. A typical initial or first burn has a reactor inlet temperature of about 650.degree. F. As the oxygen burns the coke, a temperature increase dependent on the oxygen concentration is experienced. The coke burns progressively "down" the reactor, the temperature of which is monitored by thermocouples. The amount of oxygen utilized for regeneration is approximately 0.1 to 4.0 volume percent oxygen mixed with recirculating nitrogen gas; due to this rather small oxygen volume percent, the time period required for the exotherm or "wave" to reach the bottom of a typical reactor filled with catalyst is several hours. Larger oxygen concentrations may not be utilized as too large a temperature increase would be produced exceeding the catalyst stability capacity and/or the metalurgical limits of the reactor. Only after the exotherm reaches the bottom of the catalyst bed does the recirculated gas begin to show traces of unused oxygen upon exiting the reactor. In order to insure that the bulk of the coke burnable at a given temperature had indeed been oxidized, the temperature is not increased until the oxygen content of the gases having passed through the catalyst is substantially the same (80 to 90% of initial) as the oxygen initial content of 0.1 to 4.0 which entered the catalyst bed.
The above description of hydrocracker catalyst regeneration is understood to be applicable to the in-place regeneration of other hydroprocessing catalyst having similar characteristics.
While in-place regenerations allow for decreased plant downtime when compared with catalyst replacement, such a regeneration exposes the catalyst to harmful substances such as H.sub.2 O, SO.sub.2 and CO.sub.2 formed during the coke oxidation.
It is an objective of this invention to provide for an improved hydroprocessing catalyst regeneration.
It is a purposes of this invention to reduce the time required for in-place hydrocracker regeneration.
It is an objective of this invention to reduce hydrocracker catalyst exposure to substances such as H.sub.2 O, SO.sub.2 and SO.sub.3 and CO.sub.2 formed during the coke oxidation.
Other and additional objectives and purposes of this invention will become apparent upon an examination of the entire specification and drawings, including the claims.